In the case of tray sealers known in practice, trays are normally conveyed by means of a conveyor system up to a sealing station. In said sealing station, the trays, which were previously filled with a product, are sealed by means of a cover film and closed in this way. If necessary, the trays may previously be evacuated and/or flushed with a replacement gas or a gas mixture. For transferring the trays from a feed belt into the sealing station, a gripper system is often used, this kind of gripper system being disclosed e.g. by DE 10 2010 027 211.6.
In the case of a different type of tray sealers of the type in question, the trays are positioned on a conveying path where they are taken hold of and advanced by pushers. These pushers are often crossbars extending transversely across the conveying path and having both their ends fixed in a conveyor chain. Thus, they are inevitably spaced at constant distances from one another, the respective distance being larger than the length of a tray in the conveying direction.
Circulating transfer systems used for conveying products, but not adapted for use in a tray sealer, are known from WO 00/48908 A1, DE 10 2009 003 080 A1 or DE 10 2010 028 333 A1.